Writing a GEOM Class

IvanVoras

Intel, Celeron, Centrino, Core, EtherExpress, i386,
i486, Itanium, Pentium, and Xeon are trademarks or registered
trademarks of Intel Corporation or its subsidiaries in the United
States and other countries.

Many of the designations used by
manufacturers and sellers to distinguish their products are claimed
as trademarks. Where those designations appear in this document,
and the FreeBSD Project was aware of the trademark claim, the
designations have been followed by the “™” or the
“Â®” symbol.

Last modified on 2016-03-28 17:30:19 by jgh.

Abstract

This text documents some starting points in developing
GEOM classes, and kernel modules in general. It is assumed
that the reader is familiar with C userland
programming.

1.Â Introduction

1.1.Â Documentation

Documentation on kernel programming is scarce — it
is one of few areas where there is nearly nothing in the way
of friendly tutorials, and the phrase “use the
source!” really holds true. However, there are some
bits and pieces (some of them seriously outdated) floating
around that should be studied before beginning to code:

The FreeBSD
Developer's Handbook — part of the
documentation project, it does not contain anything
specific to kernel programming, but rather some general
useful information.

The style(9) man page — for documentation
on the coding-style conventions which must be followed for
any code which is to be committed to the FreeBSD
Subversion tree.

2.Â Preliminaries

The best way to do kernel development is to have (at least)
two separate computers. One of these would contain the
development environment and sources, and the other would be used
to test the newly written code by network-booting and
network-mounting filesystems from the first one. This way if
the new code contains bugs and crashes the machine, it will not
mess up the sources (and other “live” data). The
second system does not even require a proper display. Instead,
it could be connected with a serial cable or KVM to the first
one.

But, since not everybody has two or more computers handy,
there are a few things that can be done to prepare an otherwise
“live” system for developing kernel code. This
setup is also applicable for developing in a VMWare or QEmu virtual machine
(the next best thing after a dedicated development
machine).

2.1.Â Modifying a System for Development

For any kernel programming a kernel with
INVARIANTS enabled is a must-have. So enter
these in your kernel configuration file:

options INVARIANT_SUPPORT
options INVARIANTS

For more debugging you should also include WITNESS
support, which will alert you of mistakes in locking:

options WITNESS_SUPPORT
options WITNESS

For debugging crash dumps, a kernel with debug symbols is
needed:

makeoptions DEBUG=-g

With the usual way of installing the kernel (make
installkernel) the debug kernel will not be
automatically installed. It is called
kernel.debug and located in
/usr/obj/usr/src/sys/KERNELNAME/. For
convenience it should be copied to
/boot/kernel/.

Another convenience is enabling the kernel debugger so you
can examine a kernel panic when it happens. For this, enter
the following lines in your kernel configuration file:

options KDB
options DDB
options KDB_TRACE

For this to work you might need to set a sysctl (if it is
not on by default):

debug.debugger_on_panic=1

Kernel panics will happen, so care should be taken with
the filesystem cache. In particular, having softupdates might
mean the latest file version could be lost if a panic occurs
before it is committed to storage. Disabling softupdates
yields a great performance hit, and still does not guarantee
data consistency. Mounting filesystem with the
“sync” option is needed for that. For a
compromise, the softupdates cache delays can be shortened.
There are three sysctl's that are useful for this (best to be
set in /etc/sysctl.conf):

kern.filedelay=5
kern.dirdelay=4
kern.metadelay=3

The numbers represent seconds.

For debugging kernel panics, kernel core dumps are
required. Since a kernel panic might make filesystems
unusable, this crash dump is first written to a raw partition.
Usually, this is the swap partition. This partition must be
at least as large as the physical RAM in the machine. On the
next boot, the dump is copied to a regular file. This happens
after filesystems are checked and mounted, and before swap is
enabled. This is controlled with two
/etc/rc.conf variables:

dumpdev="/dev/ad0s4b"
dumpdir="/usr/core

The dumpdev variable specifies the swap
partition and dumpdir tells the system
where in the filesystem to relocate the core dump on
reboot.

Writing kernel core dumps is slow and takes a long time so
if you have lots of memory (>256M) and lots of panics it
could be frustrating to sit and wait while it is done (twice
— first to write it to swap, then to relocate it to
filesystem). It is convenient then to limit the amount of RAM
the system will use via a
/boot/loader.conf tunable:

hw.physmem="256M"

If the panics are frequent and filesystems large (or you
simply do not trust softupdates+background fsck) it is
advisable to turn background fsck off via
/etc/rc.conf variable:

background_fsck="NO"

This way, the filesystems will always get checked when
needed. Note that with background fsck, a new panic could
happen while it is checking the disks. Again, the safest way
is not to have many local filesystems by using another
computer as an NFS server.

2.2.Â Starting the Project

For the purpose of creating a new GEOM class, an empty
subdirectory has to be created under an arbitrary
user-accessible directory. You do not have to create the
module directory under /usr/src.

2.3.Â The Makefile

It is good practice to create
Makefiles for every nontrivial coding
project, which of course includes kernel modules.

Creating the Makefile is simple
thanks to an extensive set of helper routines provided by the
system. In short, here is how a minimal
Makefile looks for a kernel
module:

SRCS=g_journal.c
KMOD=geom_journal
.include <bsd.kmod.mk>

This Makefile (with changed
filenames) will do for any kernel module, and a GEOM class can
reside in just one kernel module. If more than one file is
required, list it in the SRCS variable,
separated with whitespace from other filenames.

3.Â On FreeBSD Kernel Programming

3.1.Â Memory Allocation

See malloc(9). Basic memory allocation is only
slightly different than its userland equivalent. Most
notably, malloc() and
free() accept additional parameters as is
described in the man page.

A “malloc type” must be declared in the
declaration section of a source file, like this:

To use this macro, sys/param.h,
sys/kernel.h and
sys/malloc.h headers must be
included.

There is another mechanism for allocating memory, the UMA
(Universal Memory Allocator). See uma(9) for details,
but it is a special type of allocator mainly used for speedy
allocation of lists comprised of same-sized items (for
example, dynamic arrays of structs).

3.2.Â Lists and Queues

See queue(3). There are a LOT of cases when a list
of things needs to be maintained. Fortunately, this data
structure is implemented (in several ways) by C macros
included in the system. The most used list type is TAILQ
because it is the most flexible. It is also the one with
largest memory requirements (its elements are doubly-linked)
and also the slowest (although the speed variation is on the
order of several CPU instructions more, so it should not be
taken seriously).

3.3.Â BIOs

Structure bio is
used for any and all Input/Output operations concerning GEOM.
It basically contains information about what device
('provider') should satisfy the request, request type, offset,
length, pointer to a buffer, and a bunch of
“user-specific” flags and fields that can help
implement various hacks.

The important thing here is that bios are handled
asynchronously. That means that, in most parts of the code,
there is no analogue to userland's read(2) and
write(2) calls that do not return until a request is
done. Rather, a developer-supplied function is called as a
notification when the request gets completed (or results in
error).

The asynchronous programming model (also called
“event-driven”) is somewhat harder than the much
more used imperative one used in userland (at least it takes a
while to get used to it). In some cases the helper routines
g_write_data() and
g_read_data() can be used, but
not always. In particular, they cannot
be used when a mutex is held; for example, the GEOM topology
mutex or the internal mutex held during the
.start() and .stop()
functions.

4.Â On GEOM Programming

4.1.Â Ggate

If maximum performance is not needed, a much simpler way
of making a data transformation is to implement it in userland
via the ggate (GEOM gate) facility. Unfortunately, there is
no easy way to convert between, or even share code between the
two approaches.

4.2.Â GEOM Class

GEOM classes are transformations on the data. These
transformations can be combined in a tree-like fashion.
Instances of GEOM classes are called
geoms.

Each GEOM class has several “class methods”
that get called when there is no geom instance available (or
they are simply not bound to a single instance):

.init is called when GEOM becomes
aware of a GEOM class (when the kernel module gets
loaded.)

.fini gets called when GEOM
abandons the class (when the module gets
unloaded)

.taste is called next, once for
each provider the system has available. If applicable,
this function will usually create and start a geom
instance.

.destroy_geom is called when the
geom should be disbanded

.ctlconf is called when user
requests reconfiguration of existing
geom

Also defined are the GEOM event functions, which will get
copied to the geom instance.

Field .geom in the g_class structure is a LIST of
geoms instantiated from the class.

These functions are called from the g_event kernel
thread.

4.3.Â Softc

The name “softc” is a legacy term for
“driver private data”. The name most probably
comes from the archaic term “software control
block”. In GEOM, it is a structure (more precise:
pointer to a structure) that can be attached to a geom
instance to hold whatever data is private to the geom
instance. Most GEOM classes have the following
members:

struct g_provider *provider : The
“provider” this geom
instantiates

uint16_t n_disks : Number of
consumer this geom consumes

struct g_consumer **disks : Array
of struct g_consumer*. (It is not
possible to use just single indirection because struct
g_consumer* are created on our behalf by
GEOM).

The softc structure
contains all the state of geom instance. Every geom instance
has its own softc.

4.4.Â Metadata

Format of metadata is more-or-less class-dependent, but
MUST start with:

16 byte buffer for null-terminated signature (usually
the class name)

uint32 version ID

It is assumed that geom classes know how to handle
metadata with version ID's lower than theirs.

Metadata is located in the last sector of the provider
(and thus must fit in it).

(All this is implementation-dependent but all existing
code works like that, and it is supported by
libraries.)

4.5.Â Labeling/creating a GEOM

the utility figures out which geom class it is
supposed to handle and searches for
geom_CLASSNAME.so
library (usually in
/lib/geom).

it dlopen(3)-s the library, extracts the
definitions of command-line parameters and helper
functions.

In the case of creating/labeling a new geom, this is what
happens:

geom(8) looks in the command-line argument for
the command (usually label), and calls a
helper function.

The helper function checks parameters and gathers
metadata, which it proceeds to write to all concerned
providers.

This “spoils” existing geoms (if any) and
initializes a new round of “tasting” of the
providers. The intended geom class recognizes the
metadata and brings the geom up.

(The above sequence of events is implementation-dependent
but all existing code works like that, and it is supported by
libraries.)

4.6.Â GEOM Command Structure

The helper geom_CLASSNAME.so library
exports class_commands
structure, which is an array of struct g_command elements.
Commands are of uniform format and look like:

verb [-options] geomname [other]

Common verbs are:

label — to write metadata to devices so they can
be recognized at tasting and brought up in
geoms

destroy — to destroy metadata, so the geoms get
destroyed

Common options are:

-v : be verbose

-f : force

Many actions, such as labeling and destroying metadata can
be performed in userland. For this, struct g_command provides field
gc_func that can be set to a function (in
the same .so) that will be called to
process a verb. If gc_func is NULL, the
command will be passed to kernel module, to
.ctlreq function of the geom
class.

4.7.Â Geoms

Geoms are instances of GEOM classes. They have internal
data (a softc structure) and some functions with which they
respond to external events.

The event functions are:

.access : calculates permissions
(read/write/exclusive)

.dumpconf : returns XML-formatted
information about the geom

.orphan : called when some
underlying provider gets disconnected

.spoiled : called when some
underlying provider gets written to

.start : handles I/O

These functions are called from the
g_down kernel thread and there can be no
sleeping in this context, (see definition of sleeping
elsewhere) which limits what can be done quite a bit, but
forces the handling to be fast.

Of these, the most important function for doing actual
useful work is the .start() function,
which is called when a BIO request arrives for a provider
managed by a instance of geom class.

4.8.Â GEOM Threads

There are three kernel threads created and run by the GEOM
framework:

g_down : Handles requests coming
from high-level entities (such as a userland request) on
the way to physical devices

g_up : Handles responses from
device drivers to requests made by higher-level
entities

When a user process issues “read data X at offset Y
of a file” request, this is what happens:

The filesystem converts the request into a struct bio
instance and passes it to the GEOM subsystem. It knows
what geom instance should handle it because filesystems
are hosted directly on a geom instance.

The request ends up as a call to the
.start() function made on the g_down
thread and reaches the top-level geom
instance.

This top-level geom instance (for example the
partition slicer) determines that the request should be
routed to a lower-level instance (for example the disk
driver). It makes a copy of the bio request (bio requests
ALWAYS need to be copied between
instances, with g_clone_bio()!),
modifies the data offset and target provider fields and
executes the copy with
g_io_request()

The disk driver gets the bio request also as a call to
.start() on the
g_down thread. It talks to hardware,
gets the data back, and calls
g_io_deliver() on the
bio.

Now, the notification of bio completion “bubbles
up” in the g_up thread. First
the partition slicer gets .done()
called in the g_up thread, it uses
information stored in the bio to free the cloned bio structure (with
g_destroy_bio()) and calls
g_io_deliver() on the original
request.

The filesystem gets the data and transfers it to
userland.

See g_bio(9) man page for information how the data is
passed back and forth in the bio structure (note in
particular the bio_parent and
bio_children fields and how they are
handled).

One important feature is: THERE CAN BE NO
SLEEPING IN G_UP AND G_DOWN THREADS. This means
that none of the following things can be done in those threads
(the list is of course not complete, but only
informative):

Calls to msleep() and
tsleep(),
obviously.

Calls to g_write_data() and
g_read_data(), because these sleep
between passing the data to consumers and
returning.

This restriction is here to stop GEOM code clogging the
I/O request path, since sleeping is usually not time-bound and
there can be no guarantees on how long will it take (there are
some other, more technical reasons also). It also means that
there is not much that can be done in those threads; for
example, almost any complex thing requires memory allocation.
Fortunately, there is a way out: creating additional kernel
threads.

4.9.Â Kernel Threads for Use in GEOM Code

Kernel threads are created with kthread_create(9)
function, and they are sort of similar to userland threads in
behavior, only they cannot return to caller to signify
termination, but must call kthread_exit(9).

In GEOM code, the usual use of threads is to offload
processing of requests from g_down thread
(the .start() function). These threads
look like “event handlers”: they have a linked
list of event associated with them (on which events can be
posted by various functions in various threads so it must be
protected by a mutex), take the events from the list one by
one and process them in a big switch()
statement.

The main benefit of using a thread to handle I/O requests
is that it can sleep when needed. Now, this sounds good, but
should be carefully thought out. Sleeping is well and very
convenient but can very effectively destroy performance of the
geom transformation. Extremely performance-sensitive classes
probably should do all the work in
.start() function call, taking great care
to handle out-of-memory and similar errors.

The other benefit of having a event-handler thread like
that is to serialize all the requests and responses coming
from different geom threads into one thread. This is also
very convenient but can be slow. In most cases, handling of
.done() requests can be left to the
g_up thread.

Mutexes in FreeBSD kernel (see mutex(9)) have one
distinction from their more common userland cousins —
the code cannot sleep while holding a mutex). If the code
needs to sleep a lot, sx(9) locks may be more
appropriate. On the other hand, if you do almost everything
in a single thread, you may get away with no mutexes at
all.